1. Field of the Invention
The present invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device having an active layer made of an InGaAlP material.
2. Description of the Related Art
In recent years, an InGAlP material, which has a maximum band gap in semiconductor mixed crystal of III-V group compound other than nitride has attached special interest as an light emitting material in a wavelength of 0.5 to 0.6 .mu.m. Particularly, a semiconductor laser, which has a double heterostructure using GaAs as a substrate and InGaAlP lattice-matching therewith as a material of an active layer and a cladding layer, can oscillate visible light having a wavelength of 0.6 .mu.m at a room temperature and can be used to various applications which are not found in a semiconductor laser of an infrared region.
Since such a semiconductor laser has a short oscillation wavelength, a small beam spot can be obtained and such a semiconductor laser can be used as a light source which can perform a high density recording in an optical disc. In this case, however, such a semiconductor laser must be stably operated with an optical output of 30 mW or more.
It can be considered that the cause for restricting the optical output of the semiconductor laser is occurrence of a kink (linearity of current--optical power output characteristic is lost and bending occurs) in a current-optical power output characteristic. If kink is generated, a transverse mode is deformed and a beam characteristic is worsened. Due to this, it is difficult to use this type of the semiconductor laser as a light source for an optical disk or the like. Therefore, there is required a semiconductor laser having high kink level in order to maintain a good beam characteristic and obtain a high optical power.
Moreover, the so-called COD (Catastrophic optical damage) can be considered as another cause for restricting the optical power of the semiconductor laser.
More specifically, an active layer absorbs the oscillated laser beam itself, thereby a electron-hole pair is generated. Then, the electron-hole pair generates heat when non-emission recombination is caused, so that temperature rises. Moreover, a positive feedback is effected wherein light absorption becomes stronger by reduction of the energy gap. Due to this, crystal fusion is caused in the vicinity of the end surface of laser having high optical-power density, so that the laser is broken. COD depends on optical power density. In other words, in a case where an amount of light, which is enclosed in the active layer, is higher or the mode width is narrower, optical-power density for COD is attained in a low optical power output and the laser device is broken. Therefore, there is required a semiconductor laser having high COD level in order to obtain high optical power.
Normally, in order to increase the COD level, the active layer is thinned to reduce optical-power density in the active layer. However, in the semiconductor laser using InGaAlP, if the active layer is thinned, it is difficult to use a material having sufficiently large energy gap as a cladding layer, which is positioned at both sides of the active layer and serves to confine an injection carrier. More specifically, if the active layer is thinned, a threshold current density increases and the carrier having high energy contributes to the oscillation, so that the band gap in the active layer seems to equivalently increase. Due to this, the difference between the active layer and the cladding layer in energy gap becomes small, and the injection carrier cannot be effectively confined. Moreover, if an operation temperature increases, the reduction of the optical output is remarkable, so that it is difficult to perform an operation with high optical power.
In addition, to make it possible to perform a stable operation for a long period of time with high output, it is necessary to lower the operating current at a room temperature or temperature higher than the room temperature. Also, a laser device having a low threshold current and high differential quantum efficiency (slope efficiency) must be realized
As means for solving the above-mentioned problems, it has been considered that a method using a QW (Quantum Well) structure in the active layer is effective. In QW structure, a quantum well layer having a thickness of a wavelength of a wave function of an electron or less is sandwiched between wall layers which have energy gap larger than that of the quantum well layer and serve as barriers against an electron in the quantum well. The QW structure comprises a single quantum well (SQW) or multiple quantum well, and accompanies with an optical guide layer for confining optical power. Since an electron state in the stacking direction is quantified, and the state density increases in a step manner, a gain against the injection current becomes large and a low threshold value can be obtained. Due to this, even if the amount of optical confining in the quantum well layer is reduced, there is a possibility that oscillation having good threshold value and temperature characteristic will be obtained. It can be considered that the reduction of the amount of optical confinement is effective in improving the level against the generation of a kink due to a hole burning or the generation of COD due to light absorption.
However, in the semiconductor laser having an active layer of the quantum well structure, the energy gap equivalently increases by the quantification of the electron state, and the oscillation wavelength is shortened. The degree of shortening the wavelength varies depending on the thickness of the quantum well layer and the sizes of hetero-barriers consisting of the quantum well layer and the wall layer. More specifically, when the quantum well layer is made of In.sub.0.5 Ga.sub.0.5 P of a thickness of 80 .ANG. lattice-matching with a GaAs substrate and the barrier layer is formed of In.sub.0.5 (Ga.sub.O.5 Al.sub.0.5).sub.0.5 P, the oscillation wavelength was shortened by about 15 nm than that of the double heterostructure laser wherein a bulk crystal of the same InGaP material is used as an active layer.
Such a shortening of wavelength is a fundamental phenomenon in the quantum well structure and can be seen also in the other material such as GaAlAs. However, in a case where such structure is applied to the semiconductor laser using InGaAsP material, the following serious problem is found.
Specifically, regarding the use of a cladding layer, which is positioned at both sides of the active layer and serves to confine the injection carrier, it is difficult to use the cladding layer having sufficient energy gap. Due to the use of the quantum well structure, if the wavelength is shortened, that is, energy gap equivalently increases, an energy gap difference between the quantum well layer and the cladding layer becomes small, thereby the injection carrier cannot be effectively confined, and a threshold current increases. The increase in the threshold current is larger than the effect of low threshold current due to the use of the quantum well structure. Therefore, the effect of the quantum well structure is not always exerted in InGaAlP material.
Moreover, in conventional, in a case where InGaAlP material in which a lattice constant changes depending on its mixed crystal composition, is used in the semiconductor laser, it was considered that the difference between the substrate and the active layer in the lattice constant must be controlled to be small during the temperature at which the crystal grows from a using temperature, that is, a room temperature. This is because if the difference in the lattice constant becomes large, a misfit dislocation is generated or the expansion of the defect due to the generation of stress is improved, so that the characteristic is easily deteriorated. Particularly, in the semiconductor laser having a high injection current density and a high optical power density, such deterioration of the characteristic considerably appears by increase in the misfit dislocation or the defects.
In the normal semiconductor laser, if the difference between the substrate and the active layer in the lattice constant (degree of lattice mismatching) .DELTA.a/a is set to .DELTA.a/a=(a-a.sub.0)/a.sub.0, it was set forth as a premise that the degree of the lattice mismatching is set to be about 0.2% or less In this equation, a is a lattice constant of InGAlP layer and a.sub.0 is a lattice constant of the substrate.
FIG. 1 is a cross section showing a schematic structure of a conventional InGaAlP semiconductor laser having a transverse mode control structure. In FIG. 1, reference numeral 10 denotes an n - GaAs, 11: n - GaAs buffer layer, 12: n - InGaAlP cladding layer, 13: InGaP active layer, 14: p - InGaAlP cladding layer, 17: p - InGaP cap layer, and 19: p - GaAs contact layer. The above semiconductor laser is structured to control a current restricting or a transverse mode by use of the ridge-shaped p - InGaAlP cladding layer 14, p - InGaP cap layer 17 formed on the ridge of the p - cladding layer 14, and p - GaAs contact layer 19 embedding these layers therein and functioning as a light absorbing layer against an emitting light wavelength (for example, see Applied Physics Letters, Vol. 56, No. 18, 1990, pp. 1728-1719, JAPANESE JOURNAL OF APPLIED PHYSICS, Vol, No. 12, 1988, pp.L2414-L2416).
In the structure of FIG. 1, if the respective mixed crystal compositions of the active layer 13, cladding layers 12 and 14 are set such that the degree of the lattice mismatching is 0.2% or less, a kink level was about 40 mW, thereby limiting the maximum operating optical output.
One of the mechanisms in the kink generation, the hole burning effect can be considered. In the transverse mode, that is, the portion having high optical power density in the active layer, induced emission is strongly performed by a recombination of an electron and a hole. Thereby, concentration of the electron and the hole is lowered, and a concentration distribution of the electron and the layer in the active layer is deformed. Due to this, a gain distribution is deformed and the transverse mode is deformed. At this time, it can be considered that it is difficult for the concentration distribution of the electron and the hole to be deformed as the diffusion length of the carrier becomes larger. In a case where the InGaAlP material is used, the diffusion length of the hole is small, and this can be considered as a main reason that the kink level is low.
FIG. 2 shows a temperature dependency of the current - optical output characteristic in the structure of FIG. 1. The active layer 13 was thinned (for example, 0.04 .mu.m), thereby realizing an operation up to the optical output 20 mW. Regarding COD optical output, high COD optical output of 51 mW was obtained. However, the optical output was considerably lowered at a high temperature of 40.degree. C. or more, a practical optical output considering an operation temperature range (around 50.degree. C.) was 10 mW and this limited the the maximum operating optical output.
As mentioned above, in the conventional semiconductor laser having an active layer formed of InGaAlP material, there was a problem in that the kink level becomes low due to the hole burning effect, thereby limiting the maximum operating optical output.
Also, if the active layer is thinned and the optical output generating COD is increased, the maximum optical output at a high temperature becomes low due to the deterioration of the temperature characteristic according to the increase in the threshold current, thereby limiting the maximum optical power output.
FIG. 3 shows a specific structure near the active layer having a quantum well structure, and an energy level state of an end of a conduction band in a case where the active layer 13 has a quantum well structure in the device of FIG. 1.
Reference numeral 13a denotes an InGaP quantum well layer (four-layered structure). Reference numeral 13b is an InGaAlP wall layer. The energy level of the conduction band corresponds to the energy gap itself in the range of Al content x expressed in In.sub.0.5 (Ga.sub.1-x Al.sub.x).sub.0.5 P, particularly, x&lt;0.7, which is a direct transition type energy gap.
FIG. 4 is a relationship among the oscillation wavelength against the thickness of the quantum well layer 13a, the threshold current, and the kink level in a case where the mixed crystal composition of each layer is set to satisfy the range (.+-.0.2%) of the above-mentioned degree of lattice mismatching in the structure of FIG. 3. The increase in the kink level due to thinning is recognized. However, as compared with the DH structure, the threshold value does not always decrease as the oscillation wavelength is shortened. Therefore, the maximum temperature at which oscillation can be performed decreases. Then, the kink level in the practical threshold current considering the operation temperature range is about 60 mW, thereby limiting the maximum optical output, which can used.
As mentioned above, in the conventional semiconductor laser having an active layer of the quantum well structure formed of InGaAlP material, there was a problem in that the maximum operating optical power output was limited due to the generation of the kink level and the increase in the threshold current derived from shortening the wavelength.